Electrolytic cell and vacuum process for filling pores in its lining



July 22, 1969 A. F. JOHNSON ELECTROLYTIC CELL AND VACUUM PROCESS FOR FILLING PORES IN ITS LINING Filed Nov. 2, 1966 A lllllilllvulllllllllllllll'l INVENTOR Arthur F Johnson United States Patent 3,457,149 ELECTROLYTIC CELL AND VACUUM PROCESS FOR FILLING PORES IN ITS LINING Arthur F. Johnson, 203 Creole Lane, Franklin Lakes, NJ. 07417 Filed Nov. 2, 1966, Ser. No. 591,534 Int. Cl. C22d 3/12 U.S. Cl. 204-67 8 Claims This invention relates to cells for the electrolytic reduction of aluminum (usually called pots) and has for its object the provision of a process for the making or conditioning of the carbon lining to decrease materially the absorption of disintegrating materials into the pores of the carbon and cracks or fissures in the lining, and also provides an improved cell.

The carbon linings of electrolytic cells have uncertain service lives varying from a few days to a thousand days or more. The carbon lining is porous, and since sodium is reduced from the cryolite in the initial electrolysis in the cell, the sodium is adsorbed into the carbon cathode lining and causes disintegration. It is well known that the carbon lining develops fissures into which cryolite and molten aluminum can enter. The aluminum may make contact with iron collector bars or with the steel shell which may eat through the shell and allow the molten fluorides and aluminum to spill out onto the cellroom floor. Such fissures can form in rammed one-piece linings formed of anthracite coal, prebaked carbon blocks, or graphite.

According to one theory currently in vogue in the industry fluorides which freeze in cracks at some depth in the lining cause a prying action akin to the frost heave of moisture in the earth under cold storage buildings and probably similar to magnetic segregation of the highest melting point ingredients in mineral deposits in veins. Whatever the precise action, it is a fact that potlinings tend to heave upwards and outwards towards the sides of the cells to the extent that the open-topped steel box which encloses the carbon lining is deformed unless strongly held together by buttresses in the form of cradles or the like and a heavy horizontal steel rim.

The bad effect of aluminum and cryolite which enter the fissures or crackes, and the adsorption of sodium into pores in the carbon have been known for years but no practical solution has been suggested or put into use.

This invention provides a practical solution to the problem which comprises impregnating the carbon lining during its formation with low melting point halides such as calcium chloride or sodium chloride or mixtures thereof with each other, aluminum chloride, or with mixtures of fluorides of lower melting point than the conventional cryolite fusion. This impregnation of the pores and fissures is a barrier to the entry of the sodium, cryolite fusion or aluminum. In accordance with the process of the invention the cell lining is put under a partial vacuum during its formation, preferably under the carbon paste during the lining, baking and starting of the cell. The chlorides have a low melting point and preferentially are sucked into the carbon cathode. Also they have a high vapor pressure which tends to expel the cryolite.

The invention will be better understood with reference to the drawings in which FIG. 1 is a vertical longitudinal cross-section through a cell of the invention;

FIG. 2 is a cross-section at 22 of the cell of FIG. 1;

FIG. 3 is an enlarged fragmentary view of a suction rod used in the cell of FIG. 1, and

FIG. 4 is a modification of the cell of FIG. 1.

The apparatus illustrated in FIGS. 1 to 3 comprises the usual rectangular steel shell having a flat bottom 1, upright sides 2 and 3 and end walls 4 and 5. The shell is ice supported on a cradle including fabricated I-beams 6 and upright braces 7 which are sandwiched between two channel bars which hold the long sides in position against bulging or warpage. The refractory alumina lining 8 is vibrated into place over the bottom 1 and between the sides 2, 3 and the ends 4, 5 and the sheet steel or refractory form 9. The cathode or potlining 10 is formed in the usual manner of a mixture of anthracite coal and tar of pitch binder. As is common in large cells, a plurality of iron current collector rods R are embedded in the carbon cathode to collect the cathode current. Before the application of vacuum all places where the atmosphere might leak into the cell, as through holes in the steel shell for collector bars or the like, are carefully sealed with refractory mortar supplemented with coats of sodium silicate paint or preferably Carbofrax paint.

In accordance with my invention, I provide the alumina layer 8 on the bottom 1 with any suitable number of perforated pipes 12 which are preferably covered with a heat-resistant, fiber fabric sock 13 as best shown in FIG. 3 so that the granular alumina as initially rammed or vibrated into place does not plug the holes H. These pipes are connected to upright header pipes 15, preferably located in the corners of the cell, and these pipes 15 are connected to a vacuum pump system (not shown) to put the pipes 12 under a low pressure.

FIG. 4 illustrates a modification of cell having a grid of expanded metal 16 covered with filter cloth embedded in the refractory layer 8 which is connected to upright pipes 15. As in FIG. 1, this grid provides a means for applying a partial vacuum to the entire alumina lining which is the means for putting the pores and fissures in the cathode lining under low pressure while it is being formed.

After the alumina lining has been packed into a dense structure by vibration, the potlining carbon paste is applied. The layers of carbon paste are rammed into the bottom of the cell on top of the alumina or other porous insulation, the vacuum is applied gradually through pipes 15 and pipes 12 or the grid 16, as the case may be, and only to the extent of a few pounds or less at first so as not to deplete the first layer of unrammed mix of the more volatile ingredients of binder pitch. The first layers of rammed mix are chilled by contact with collector bars unless these are thoroughly preheated to paste temperature which is recommended in the practice of this invention. After the first layers of carbon paste are rammed, these layers act as a plug on which higher vacuum can be drawn to gently but firmly consolidate the entire mass by the combination of atmospheric pressure and applied vibration. The effect of the atmospheric pressure on top of the lining caused by vacuum beneath the lining during baking of the lining is to close the voids resulting from loss of volatiles or shrinkage of the aggregate of carbon particles. At the time when the carbon lining is being con solidated during the tamping and baking process the granular alumina insulation is also consolidated by the tamping vibration and by the applied vacuum. A granular solid, like alumina, tends to compact best by vibration without vacuum because vacuum tends to bind all the particles together preventing their free flow. Hence, during tamping of the carbon mass vacuum should be cut ed and on enough times so that the insulation is compacted as much as possible. During baking of the lining it will be found advantageous to continue to vibrate the alumina side insulation down and add fresh alumina thereto to make up for the shrinkage of the carbon potlining mass during baking. In other words, in the practice of this invention it is necessary to crowd and compact the potlining from all sides including the lateral sides as well as the top and bottom to minimize the number of fissures and the porosity in the carbon potlining. It is important also to use a very strong cradle and shell reinforcement to minimize 'expansion and distortion of the cell. Preferably the reinforcement should be prestressed by making the shell at room temperaturefit snugly in between the cradles after these have been preheated to between 100 F. and 400 F. depending on the strength of steel used in the cradles. The preheating of the cradles may be done with gas burners just before the steel pot shell (at room temperature) is dropped between the cradles. The pot shell will ordinarily operate at a bottom temperature of 150 to 350 F. depending principally on the amount of insulation used, amount of air circulation permitted under the shell and cradles and the temperature of the circulating air. As the shell warms up to operating temperature during baking it should highly stress the cradles, but, of course, not exceed their yield strength. Preferably the shell should be preheated before the potlining carbon is rammed into place so that it will not expand away from the potlining carbon and heat insulation and leave the carbon mass unstressed. When such preheating of the shell is not done before ramming the carbon paste, greater care must be taken to add granular alumina insulation around the sides of the shell and vibrate this into all open spaces formed between the shell and the carbon mass as the shell expands and as the carbon mass contracts during baking.

Where the carbon cathode lining is built of prebaked carbon blocks (which are much more expensive than a rammed mass) the same general procedure and precautions outlined above are followed excepting much less paste is required to cement the blocks together than to ram the mass in one piece. Vacuum can be employed very effectively in ramming and baking the joints uniformly to a high degree of compaction so that failures do not occur at the joints as much as in conventional practice.

Whether the potlining be made entirely of one mass or rammed lining or be built of prebaked blocks with paste rammed between, the next step after the lining has been baked out with electric current comprises applying to the surface of the carbon lining a coating or layer of molten chlorides such as calcium chloride, magnesium chloride, or sodium chloride to which may be added aluminum chloride, or with mixtures of fluorides. For example, the material applied to the lining may comprise:

(a) a mixture of 62% NaCl and 38% MgCl having a melting point of about 450 C.

(b) a mixture of cryolite and Allso that the percentage of All-' is about 47% and the NaF is about 53% having a melting point of about 700 C.

(c) a mixture of AlF NaF and CaF in about equal molecular proportions which has a melting point of about 820 C. The NaF may vary widely or be omitted without greatly changing the melting point.

It is advantageous that the mixtures be premelted to assure their low melting points when applied.- 7

The fused halides of low melting point are drawn into the carbon by applying a vacuum by means of pipes 12 or the grid 16 of FIG. 4. The vacuum should be applied as soon as the molten chloride layer is added and before any molten aluminum has been poured into the cell cavity or any appreciable amount of aluminum has been electrolyzed therein. It has been determined by experiment that carbon at the same temperature of molten electrolyte can be rendered gas-tight by drawing a vacuum on the carbon immersed in the electrolyte. The next step in the operation is to pour molten cryolite fusion into the cell, say to a depth of eight inches, and then a small amount of molten aluminum. It is important that the side lining be baked as thoroughly and heated as hot as the bottom before themolten electrolyte is poured in to seal the pores with the aid of vacuum drawn under the lining. Ordinarily the side lining is not as hot as the bottom lining because the anode electrodes do not touch it. An important part of this invention is heating the side lining as well as the bottom of the carbon mass by directing; part of the bakingcurrent through' the"side.--This can be done by wedging a carbon block or blocks between the side lining and one of more anodes at a time while breaking the contact in Whole or in part between such anodes and the bottom lining by very slightlylifting such anodes from the bottom. It is important that all exposed carbon potlining be sealed with halides or other suitable substance and preferably at the same time because otherwise a strong vacuum cannot be drawn under the lining and there is less tendency for the halide to be drawn into the small fissures and pores of the carbon mass, Instead of fused chlorides, fused fluoride mixtures with low melting points may be used to impregnate the carbon lining. In any case the impregnation should preferably be done with an acid fusion, that is, in the case of fluorides, one which consists of cryolite plus aluminum fluoride rather than cryolite alone. The reason is that such impregnation if acid will react with adsorbed sodium and hold it from further penetration into the carbon mass. Also from the best information thus far obtained the viscosity of molten cryolite at 1000 C. is about 6.8 cp. compared with 4.8 cp. for cryolite plus 15% aluminum fluoride so the latter will be more easily absorbed and especially since such acid electrolyte has lower surface tension.

Conventional practice has been to start new aluminum reduction cells with cryolite or alkaline bath and keep it alkaline by soda ash addition because the rapid absorption of sodium by new carbon linings quickly made the electrolyte highly acid with remaining excess aluminum fluoride. According to this invention in the very first minutes of starting electrolysis an electrolyte high in aluminum fluoride (acid bath) is used with the aid of applied vacuum under the lining to seal the potlining surface, then soda ash additions can be greatly reduced if not eliminated entirely with only neutral cryolite and calcium fluoride being added to the electrolyte bath.

Molten electrolyte made of fused fluorides is expensive costing in the order of $200 to $300 per ton whereas molten calcium and sodium chloride cost only 15% to 20% of this amount, hence the latter are preferable for impregnation of the carbon lining but are also recommended because of lower melting points preventing the above mentioned heaving of the lining due to freezing therein.

In any case the low-melting halides used for impregnation may be applied to the red hot potlining surface as a powder as well as in molten 'form providing the powder has been previously melted and crystallized as a low-melting mixture. If the pot side lining is prebaked, it may be helpful to superficially seal its exposed surface with a mixture of sodium silicate and pulverized cryolite. Then after a low-melting fusion has been used to impregnate the surface of the bottom of the potlining cavity, a low-melting fusion may be applied to the side lining surface so as to dissolve the superficial sodium silicate coating and be drawn into the body of the side lining by the applied vacuum.

. After electrolysis has been started by gradually raismg the anodes, granular or molten cryolite is added with additions of aluminum fluoride or other halides to lower the melting point until, as quickly as possible, the entire pot cavity is filled. Thereupon the pot cavity surface is more completely sealed and a higher vacuum can be achieved to impregnate the carbon mass with low melting fusion. When a high vacuumis held with little further pumping required, no further impregnation is occurring and the low melting fusion may be transferred to another pot or to storage and the fluorides for conventional electrolyses may be'added and utilized in the conven tional manner.

I have found, in the course of my investigations, that when carbon such' as coal mixed with a binder suchas pitch is heated while under'a vacuum to form either a dense carbon such as a cathode cell lining, or to form graphite, instead of expelling the carbonaceous volatiles,

they are drawn itno the pores where they carbonize and increase the density and reduce the porosity.

I claim:

1. The process for filling the pores and fissures of carbon cathode linings of electrolytic cells for the reduction of aluminum which comprises applying vacuum to the area between the shell and the carbon cathode while it is being heated and baked, and thereafter providing within the cathode a molten halide material of relatively low melting point which is sucked into the pores and fissures thereby excluding the entry of the relatively higher melting point cryolite fusion used during subsequent electrolysis.

2. In the process of claim 1 charging the cell with a mixture of alumina-cryolite fusion and one of the chlorides of sodium, calcium, magnesium or aluminum to lower the melting point of the fusion and facilitate its entry into the pores and fissures of the cathode lining.

3. In the process of claim 1' applying to the carbon cathode one of the halide salt mixtures: NaCl about 62% and MgCl about 38%; AIR NaF and CaF or cryolite and MP3.

4. In an electrolytic cell for the reduction of aluminum having a steel shell, a refractory lining inside the shell sides, ends and bottom, a carbon cathode inside the refractory lining, the improvement which comprises means between the shell and the carbon cathode to apply a reduced pressure and draw into the pores or fissures of the carbon cathode a sealing material.

5. The cell of claim 4 in which a perforated member is embedded in the refractory lining on the shell bottom and connected to a vacuum means.

6. The cell of claim 5 in which the perforated member is covered with a filter fabric.

7. The cell of claim 4 which comprises a grid between the shell bottom and the refractory which provides a space to apply a vacuum over the bottom of the refractory, and a vacuum system in operative connection with the space.

8. The cell of claim 7 in which the grid is covered with a filter fabric.

References Cited UNITED STATES PATENTS 2,471,330 5/1949 Knight et al. 11761 2,662,930 12/1953 Morelock 117-61 XR 2,959,533 11/1960 De Varda 204--67 3,120,454 2/1964 Bailey et a1 11761 3,293,406 12/1966 Bain 20467 JOHN H. MACK, Primary Examiner D. R. JORDAN, Assistant Examiner US. Cl. X.R. 

1. THE PROCESS FOR FILLING THE PORES AND FISSURES OF CARBON CATHODE LININGS OF ELECTROLYTIC CELLS FOR THE REDUCTION OF ALUMINUM WHICH COMPRISES APPLYING VACUUM TO THE AREA BETWEEN THE SHELL AND THE CARBON CATHODE WHILE IT IS BEING HEATED AND BAKED, AND THEREAFTER PROVIDING WITHIN THE CATHODE A MOLTEN HALIDE MATERIAL OF RELATIVELY LOW MELTING POINT WHICH IS SUCKED INTO THE PORES AND FISSURES THEREBY EXCLUDING THE ENTRY OF THE RELATIVELY HIGHER MELTING POINT CRYOLITE FUSION USED DURING SUBSEQUENT ELECTROLYSIS. 